A typical extrusion press for making profiles of nonferrous metal has at least one ram driven by at least one piston/cylinder actuator in turn supplied with hydraulic fluid under pressure by at least one pump. This pump is driven by an electric motor connected to a multiphase line or power source.
Such an extrusion press is known from EP 2 000 226. Here, a three-phase asynchronous motor is used to drive the pump. For a soft start-up of such powerful motors, such as in particular three-phase motors, it is known to hook them up in a star-delta connection. The star-delta connection is used to limit the starting current of an asynchronous motor in delta connection. Here, the motor is brought to speed in the star connection. When switching over, theoretically, only the delta current is necessary to get to the nominal speed. Thus, the starting current is reduced to a third the current in a direct start-up in delta connection. However, when switching from star to delta connection, the phases of the line current and the motor field can be in opposition to each other. This results in equalization processes that disadvantageously lead to very high peak switch currents.
In particular, when using the star-delta switching and a fault-voltage protection switch, it is not possible or only limitedly possible to run a cyclical pump switching circuit in an energy-efficient manner.
As is the case for fault-voltage protection switches integrated in the motor control, a frequent switching off is afflicted with problems or even impossible. This is the reason why in generic plants, the pumps that are not needed run is continuously because their motors cannot be switched off. This results in energy-related disadvantages because the pumps are operated to a greater extent than it is necessary for carrying out the pressing process.